1. Field of the Invention
The present invention relates to a frequency finding circuit for finding a frequency band of an input signal, and more particularly, it relates to an apparatus for processing input signals having different frequency bands.
2. Description of the Prior Art
FIG. 9 shows an architecture of a prior art frequency finding circuit 103. An input signal of which frequency is to be found is input to an input terminal 11. For reasons, for example, that a frequency of the input signal must be found for a period when the frequency is relatively stable, a signal sampling circuit 21 obtains values of the input signal for a specified period (sampling period) in accordance with a sampling signal G input on a terminal 13; that is, it performs-sampling. A sampled input signal S is converted by a frequency discriminator 30 into a d.c. signal K having a value corresponding to the frequency of the input signal only for the sampling period. The signal K transmitted through a sample/hold circuit 24 is compared with a specified reference voltage by a voltage comparator 25. A result of the comparison is applied as a deciding signal to an output terminal 12.
The frequency discriminator 30 is comprised, for example, of a resonance circuit 34, a phase shifter 23 for shifting a phase of the input signal by a difference between a resonance frequency of the resonance circuit 34 and a frequency of the sampled input signal S, and a phase detector 22 for receiving the resultant phase from the phase shifter 23.
The prior art frequency finding circuit is configured and performs the sampling of an input signal in such a manner as mentioned above, and therefore, an output from the frequency discriminator 30, namely, the signal K, is significant only for the sampling period. Thus, applying the signal K in raw data to the comparator 25 cannot allow the deciding signal applied to the output terminal 12 to represent the frequency of the input signal. This is why the sample/hold circuit 24 must sample the output from the frequency discriminator 30 in accordance with the sampling signal G to obtain values contained by the signal K, and thereafter apply to the voltage comparator 25.
In this stage, in order to gain a stable result, it is desirable for the sample/hold circuit 24 to have a larger time constant. For that purpose, the sample/hold circuit 24 must have a capacitor 34 connected through an external terminal 42. However, integration of the capacitor 34 is difficult, and an existence of the external terminal 42 may prevent an enhancement of a capability of the integration; that is, the prior art frequency finding circuit has the problem that its integration is difficult.